I set my price to 3, 2.99 and 2. I get happy and unhappy customers but my money never grows.
I just released an update, so that may have fixed the problem. If you have lemonade and you have cups then you should be selling lemonade. I would start lower in price @ around $1.00 as the reputation is what increasing the customers spending. I would make sure your price is lower, you currently have lemonade made, and you have cups as each lemonade sale needs one. The angry faces mean that something is missing and the customer is upset about it and it will affect your reputation, while the orange face means they are indifferent to the sale and it can affect your reputation though not every time. I may address the notifications in the next update to give more info instead of emojis.